Various types of fiber-based optical amplifiers, such as erbium-doped fiber amplifiers (EDFAs) and distributed Raman amplifiers (DRAs), are ubiquitous components of optical communication systems, eliminating the need to perform optical-electrical-optical signal transformations when regeneration of a fading optical signal is required.
In the case of EDFAs, an optical pump laser (typically operating at 980 nm) is coupled into a section of Er-doped optical fiber, and the incoming optical signal is propagated through the doped fiber with the pump light. The presence of the pump light with the erbium dopant generates amplification of the propagating optical signal by the transitions of the optically-excited erbium ions. Distributed Raman amplifiers (DRAs) operate by injecting short, high-power pulses along a section of transmission fiber that is supporting the propagation of an optical signal. The presence of these pulses (either co-propagating or counter-propagating with respect to the optical signal) excites the photons to higher energy levels, where the photons create stimulated emission as they return to their ground state.
The various components forming an optical amplifier module are typically made as fiber-coupled elements, and in some cases integrated (or hybridized) to form, for example, a combined isolator and WDM filter, or a combined isolator and GFF filter, or the like. Of course, lower cost and smaller-sized modules lower the overall system costs. Thus, the trend to smaller components, more hybridization and smaller modules has been taking place for some time. Indeed, the pressure for smaller form factors and lower costs continues to be exerted on the industry.
One path to assuage these demands is to continually reduce the size of the various components and, perhaps, increase their degree of integration. However, this is not easily accomplished in an environment where the cost of the amplifier module is also a concern. Indeed, the size of these components has decreased to the point where they cannot be readily assembled by low-cost labor (i.e., the size of some of these components can be on the order of 1 mm×1 mm×1 mm).
Furthermore, even with reduction in size of an optical amplifier module, such as from increasing the level of integration within the hybrid components, the different hybrids must be coupled to each other via fiber splicing and routing. As a consequence of the minimum bend radius of the optical fiber as well as the relatively large number of fiber splices and splice protectors mandating the same, the ability to further hybridize current configurations is quickly reaching its technical limits, size limits and economical possibilities of implementation. The “bend radius” is a determinative factor associated with defining an acceptable amount of signal loss. In particular, the loss exhibited by an optical signal increases with a smaller bend radius of the fiber in which the signal is propagating. At exceptionally small bend radius values, there may also be a physical failure of the fiber itself.
Thus, for a fiber-based optical amplifier to continue to meet the expectations of cost and size reduction, while maintaining performance requirements, a different approach to incorporating the amplifying fiber within the optical amplifier module appears to be required.